Obviously, safety is paramount when you’re working on control systems (or the machines under their control). This is true for many reasons. Aside from potential injury, accidents can directly and indirectly affect both you and your company.
Personnel safety. On whatever control systems you design, install, or maintain, it’s critical you incorporate guards, shields, monitoring devices, and interlocks in every applicable location. You should worry about the safety of everything and everyone associated with it.
The first level of maintaining operator safety is to stop the motion of any moving mechanism having the potential to injure any portion of a worker’s body. Here, you connect the safety device to the control circuit like a second stop button. That is, you locate it in a series portion of the motor control circuit, just like the usual stop button. Obviously, this must be a normally closed (NC) device. When the device senses a worker’s presence (or any other object) in the danger zone, the control circuit opens, shutting down the operation. But pay attention here: Some processes don’t shut down quickly. In such cases, you must have other plans. One example is a plugging circuit, which momentarily reverses current to the motor, stopping it quickly. The type of switched device depends on the manner of activation.
When an operator approaches a moving mechanism, he or she could:
Break the light beam of a photoelectric switch;
Move a gate that activates a limit switch; or
Move off of a footpad that activates a pressure switch.
Other common types of devices used for safety circuits include ultrasonic switches, proximity switches, and push buttons with oversized activators.
There are two primary methods for using electrical controls in safety circuits:
Circuits that activate a guard, which prohibits the operator from reaching into the danger zone at any time (machine stopped or running).
Circuits that immediately stop the machine when any portion of the operator’s body is in the danger zone.
There are three types of safety control techniques. One is installing a multiple stop device in series. A second is installing a redundant stop device in case of failure of one device. The third is installing an interlocked control circuit, which inserts an NC contact from one device’s controls into the control circuit of another. This ensures device A will not operate while device B is energized.
You usually design safety circuits specifically for the shape and geometry of the machine, where the operator is in relation to the machine, the speed of the mechanism, and other factors you can determine only after watching how the operator moves and works. As you can see, safety is a continuous process you must constantly refine and improve. The way people use machines changes over time. The way they receive materials, how they’re loaded and unloaded, and what their duty cycles are may change.
During the design stage, you should think about the operation of the machine, but you may not be able to foresee some of the fine operational details that will occur through years of use.
When you’re installing a control system, you’ll spend a great deal of time in the areas where operators use these control systems. Hopefully, you’ll gain some familiarity with the people who run the machines. Here’s where you may be able to notice safety issues during the design stage.
When maintaining a control system, you’ll notice changes to it as well as the machine itself, its process, and the operator working with your system. In any case, and at whatever time you see a potential safety problem, it’s your responsibility to take care of it.
Facing reality. The safety of systems we install is a serious issue, affecting peoples’ bodies and lives. Machines are dangerous, and no matter how hard we try to eliminate all accidents, they still happen. If you are to be involved with machine controls, it is critical to you (for your own personal benefit) to consider the real situations you face.
This issue, more than any other we’ll cover in this course, deserves plain-spoken coverage.
The first common safety problem many of us face is that of management not wanting to spend the necessary money. Let me begin by saying that almost any manager in the world would be sickened by one of their employees being maimed or killed. Safety problems that you may have with management are not going to be that they don’t care about people being maimed; they will be that they do not understand the gravity of the situation. Production pressure is foremost in their minds, and “adding a guard to a machine” just doesn’t stick out.
That means it’s your responsibility to get the point across. If you design, install, and maintain control systems, you are in a highly-responsible positions—you’ll have to be ready to make a stand.
If you notice an acutely unsafe situation (capable and likely to cause injury at the present time), it’s your responsibility to eliminate it immediately. It is your job to make an intelligent decision quickly. If you have to shutdown a machine or assembly line, do it. It is your responsibility to everyone who works on that equipment to keep them safe. If taking such an action will result in your job being threatened, you must do it anyway. You are responsible, and you must be able to exercise that responsibility. If you cannot or will not—find a different position.
If you run into a situation where there is a concern, but that you don’t think is likely to cause injury right away, try to eliminate it as quickly as you can, without creating a disturbance in your company’s operation. If it requires extra resources, go to your manager and explain the situation. In most cases, the manager will tell you to get the job done right away. In the rare instance where the managers do not want to do the work, explain it again, and try to make them understand the seriousness of the matter. If they still refuse, it is your obligation to quit your job immediately. If you can’t stand up to this type of choice—do a different job.
Safety programs. Don’t rely on the current fad of safety programs—in general, they’re designed and promoted by consultants and quasi-governmental activists who end up getting a piece of the action somewhere along the line. Any program that gives you a template of exactly what you should inspect, write down, and report is suspect. Not because they are telling you to do wrong things, but because they are telling you how to think. Take whatever good ideas they have (and they do have some), and make the reports that your employers ask you to; but never allow someone else to do your thinking for you.
Take whatever good ideas your safety plan has, and follow your instructions, but never, ever, let the “plan” substitute for your own good sense and diligence.
What if? Although few of us will ever come face-to-face with this, that fact is that accidents can and do happen to people who work on our machines.
This is an ugly issue, but an important one, especially if it happens to you. There are three critical time periods surrounding any accident you are involved with:
1. Preparation before the accident occurs.
2. During the emergency situation.
Before the accident occurs: If the possibility of an accident exists, as it does in our business, it makes sense to prepare yourself for an accident. Face the fact that such a thing could happen. It is easy to ignore such an ugly subject as a serious accident, but you must not, for two reasons:
First, because turning away from the subject will prevent you from recognizing some preventative actions. Once you decide that accidents can happen, but that you won’t let them happen, you will be more effective at preventing accidents.
Secondly, because facing the subject head-on will get you prepared to get through the trauma of an accident, should one really occur. If you can honestly say (to yourself) that you did consider the possibility as well as you could, you’ll fare much better.
Bear this in mind—You will never be able to prepare perfectly for safety. You do not have perfect knowledge. Neither can you spend all of your time and energy on accident prevention only—that is not realistic. But you should do enough preparing now, so that if something bad does occur, you won’t have reason to be angry with yourself. Once in a while, take a walk through your area with the goal of spotting possible trouble spots. Most likely, you will not find any. But an occasional tour will help you spot anything that is a real problem.
Don’t invent problems, and don’t expect to eliminate every possible risk—that’s impossible. Your goal is to keep your systems free of reasonably avoidable risk.
In all your analysis of safety, you will be making judgements as to what is an acceptable risk and what is not. Your judgement is not perfect, nor should you expect it to be. But it should be reasoned. Beyond that you can do no more.
During an accident: If an accident does occur, you must make sure that you take care of first things first. In many ways, facing an industrial accident is very similar to facing a combat situation: a mixture of horror, shock, and disorientation. In such a situation, your perceptions of time and distance are altered, and your focus narrowed to a sort of tunnel-vision.
Under such circumstances, you will need to keep your actions as simple as possible. Do the really important things first—shutting down the machines, calling the paramedics, and administering first aid (or finding someone who can). After that, you can start to think about other things. Keep it very simple.
Few of us can handle such things very well, and most of us get physically ill during or afterwards. You cannot help this—it’s simply the way that human beings are.
Again, should an accident occur, keep it very simple, and take care the few really important things that only you can do (such as shutting down the electrical equipment). After you finish that, you can take care of other things. And expect to experience some sort of physical and psychological shock—that is just what happens to us. Immediately after the accident, let yourself wind-down and come back to normal slowly—it will probably take several hours.
Afterward: After an accident, your problems will be emotional. You will almost certainly feel guilty in one way or another. Again, this is normal. Your goal at this point is not to let feelings of guilt get out of hand. Think about it reasonably, and accept no unearned guilt. Guilt is so deeply conditioned in most of us that it can be a very powerful force. Think about it rationally, and not just emotionally.
If you feel bad about the accident for very long, get some stress counseling. Getting counseling does not mean that you are weak—it means you’ve been through some really tough stuff, and you have sense enough to get some help from someone who has been through it before. Getting counseling does not mean that you are weak - it means that you’ve been through some really tough stuff, and you have sense enough to get some help from someone who has been through it before. Take the help.
The safety of the mechanism. You should also consider safety at points physically away from the actual mechanism but ones that still affect the movement of the mechanism. Examples include the circuit breakers that control the electrical power to the machine and the push button controls for the machine. When maintenance personnel work within the danger area of a machine, they should always disconnect the power and lock the control device with a key. A person responsible for the safety of the workers in the area usually holds this key, which also unlocks and reactivates the power.
Under the law, an employer must provide employees with a safe place to work and rules for them to perform their work safely. The federal government enacted the Occupational Safety and Health Act, which created The Occupational Safety and Health Administration (OSHA) as an agency within the Department of Commerce. This group is responsible for monitoring the safety of workers and prosecuting any company violating the agency’s policies. Besides OSHA, other organizations are also involved in worker safety, such as insurance companies, industry-specific agencies, and local and state government agencies.
Machine safety needs. Many machines designed today are capable of damaging other machines. For example, in complex manufacturing work cell designs, many independent machines move in and out along controlled paths that intersect the paths of other machines. If any one of the machines stops or malfunctions, others could crash or become entangled with the defective one. This type of industrial accident would not involve an employee, but damage to expensive equipment could stop a company’s entire production line.
Today’s articulated machines, such as robots and coordinated machines such as machining centers, need protection from themselves and other machines. For example, in machining cells, robot arms frequently pass through the line of travel of adjacent machines.
Machine safety in a coordinated and programmed manufacturing system involves the use of detectors and programmed safety sequences. Examples of common detectors include:
Zero-speed switches (for detecting stopped motors);
Over torque switches (for detecting jammed parts);
Proximity switches (to detect over-travel);
Flow switches (to detect loss of lubricating oil);
Pressure switches (to detect jammed hydraulic actuators); and
Temperature switches (to detect overheating).
Safety sequences are very difficult to put into place, because each machine should work independently. Therefore, when one machine joins a coordinated work process with others, there’s sometimes a need for a master controller. This controller is not only responsible for the correct sequencing of each machine in the process pattern, but also for the recognition of a problem, determination of the severity of the problem, and the best corrective action. The corrective action is in the form of a programmed sequence of moves given to each machine in the process. The sequence involves the following actions:
Stop all machines;
Evaluate the current position of each machine;
Begin to retract each machine to a home position;
Notify the operator of the detected problem;
Identify the defective machine and source of the problem; and
Wait for a restart command from the operator.
Troubleshooting. Always turn off all power and use lockout/tagout procedures in any situation where you must come in contact with the circuit or equipment. Make sure no one but you can turn on the equipment.
Use only well-designed and maintained equipment to test, repair, and maintain electrical systems and equipment. Use appropriate safety equipment such as safety glasses, insulating gloves, flash suits, hard hats, insulating mats, etc.
Effective troubleshooting starts with analysis of the problem. Breaking it down into sections limits the size of the job. Considering categorizing the following areas:
Electrical or electronic;
In many cases, the problem may be a combination of two or more of these areas. For example, problems in the electrical contacts or the mechanical operator may cause a limit switch to not function properly.
As long as people operate machines, problems will arise that do not respond to the usual form of troubleshooting. These problems may stem from misunderstanding, lack of cooperation, or lack of knowledge of the machine. Whatever the cause, you should handle them carefully and diplomatically so you quickly can return the machine to its intended job.
You can further separate problems into physical location or type of operation. For example, you may find a problem localized in only one section of a complex machine. This immediately eliminates the rest of the machine as a possible trouble source. Success in troubleshooting frequently lies in the ability to segregate the problem area from other unrelated circuitry.
Here are the common problem spots.
Blown fuses. You must eliminate the reason for the overload and replace the fuse with the proper type.
Loose connections. There could be dozens of connections on a given machine. Each of these spots may be a source of trouble. A loose connection in a power circuit can generate local heat, which spreads to other parts of the same component, other components, or conductors. One example of where direct trouble can arise is in thermally sensitive elements, which can be overload relays or thermally operated circuit breakers.
Faulty contacts. Such components as motor starters, contactors, relays, push buttons, and switches apply here.
Problems in the NC contact are of the most difficult to locate. A contact may look closed, but still not conduct any current. Check any contact that has had an overload through it for welding. Weak contact pressure, dirt, or an oxide film on the contact surfaces will prevent it from conducting.
Many times, you can clean contacts by drawing a piece of rough paper between them. Use only a fine abrasive to clean contacts, and do not file them. Most contacts have a silver plate over the copper. If you destroy this by filing, the contact will have a short life. If a fine abrasive will not clean them, it’s better to change them out.
Another problem occurring with double-pole, double-break contacts is cross-firing. That is, one contact of the double break travels across to the opposite contact, but the other remains in its original position. If you’re using both the NO and NC contacts in the circuit, a malfunction of control may occur.
Incorrect wiremarkers. This problem usually appears on the builder’s assembly floor or in reassembly in the user’s plant. The error can be difficult to locate, as a cable may have many conductors running some distance to various parts of the machine.
Combination problems. Typically, these problems can be electrical-mechanical, electrical-pressure (fluid power or pneumatic), or electrical-temperature.
The greatest problem is not always indicative of which aspect is at fault. It may be both. It’s usually faster to check the electrical circuit first; however, you must check both systems. For example, few solenoid coils burn out due to a defect in the coil. Most solenoid troubles on valves develop from a faulty mechanical or pressure condition that prevents the solenoid plunger from seating properly. This condition causes the solenoid to draw excessive current. The result is an overload or burned-out solenoid coil.
Low voltage. If no immediate indication of trouble is apparent, one of the first checks to make is the line and control voltage. Due to inadequate power supply or conductor size, low voltage can be a problem. This problem generally shows up more on starting or energizing a component, such as a motor starter or solenoid.
Heat is one result of low voltage you may not notice immediately in the functioning of a machine. As the voltage drops, the current to a given load increases. This produces heat in the coils of the components (motor starters, relays, solenoids), which not only shortens the life of the components but may also cause malfunctioning. For example, heat can cause moving metal pans with close tolerances to expand to the point of sticking.
Grounds. There are many locations on a machine where a grounded condition can occur. However, there are a few spots in which grounds occur most often, including:
Connection points in solenoid valves, limit switches, and pressure switches. Due to the design of many components, the space allowed for conductor entrance and connection is limited. As a result, a part of a bare conductor may be against the side of an uninsulated component case.
- Raceway openings.
When pulling conductors through conduits or into pull boxes and cabinets, you can scrape or cut the conductor insulation. If you don’t eliminate sharp edges or burrs on freshly cut or machined parts, cuts and abrasions can occur.
- Loose strands.
The use of stranded wire greatly reduces many problems in machine wiring. However, you must be careful when placing a stranded conductor into a connector: You must use all the strands. One or two unconnected strands can touch the case or a normally grounded conductor, creating an unwanted ground. Even if the ground condition does not appear, you reduce the current- carrying capacity of the conductor.